Thermal picture synthesis devices are used for generating thermal images for use in developing and testing infrared radiation sensitive electro-optical equipment. Conventionally such a device may take the form of one or more resistor elements, preferably arranged in an array, which can be heated selectively by passing electric current therethrough. By selecting the resistor elements to be heated the device can be used to produce a desired thermal image made up of pixels which correspond to the heated resistor elements.
A form of thermal picture synthesis device has been proposed which employs a resistor element in the form of a thin strip extending across a cavity formed in the surface of a supporting semi-conductor substrate. The resistor element is also made of silicon, is suspended over the cavity and is connected to drive circuitry arranged along side and coplanar with the resistor element on the semi-conductor substrate. By suspending the resistor element over a cavity it is possible to reduce the rate of loss of heat from the resistor element into and through the semi-conductor body. This allows more of the heat generated in the resistor element to be used for the thermal picture synthesis. However such a known device suffers from at least two problems which reduce the usefulness of the device.
Firstly by making the drive circuitry coplanar with the resistor elements the number of elements or pixels per unit area in an array is reduced. This reduced "fill factor" leads to a large loss of apparent temperature so that the actual perceived thermal image temperature may only be of the order of 300.degree. Centigrade whilst the actual temperature at the resistor element is of the order of 1100.degree. Centigrade. Secondly this large loss of apparent temperature is exacerbated by the use of a silicon resistor element which can additionally lead to a high thermal stress failure rate for the device. Because of these problems such a known form of thermal picture synthesiser device is not suitable for imaging temperatures of more than about 300.degree. Centigrade and there is a need for a generally improved thermal picture synthesis device which is capable of operating at higher apparent temperatures, preferably up to 600.degree. or 700.degree. Centigrade.